1. Field of the Invention
The present invention relates to apparatus for applying time base corrections to video signals.
2. Description Relative to the Prior Art
When transmitting video signals, particularly where record/playback operations are employed, it is a problem to maintain a time base consistent with a standard format for video signals, for example, the conventional NTSC signal format. Time base departures from the standard, say the 63.5 .mu.sec. period over one line of a video frame with the NTSC standard, may occur for a wide range of frequencies and are generally referred to as signal flutter.
A TV set typically tolerates low-amplitude flutter at low frequencies, employing for such purpose circuits which loosely slave the TV display to the time base of the received signal. (To lock the display tightly to the signal time base would introduce objectionable instabilities.) In the case of record/playback systems phase-lock-type speed controls can, for practical purposes, eliminate low frequency flutter, viz flutter at frequencies below the video field frequency. Higher frequency flutter, which may result from such causes as recording-tape stretch variations and transmission-time variations, proves more difficult to avoid. Such flutter, moreover, is not easily compensated for in the signal processing of a TV set, and can result in picture distortions or other irritating display problems.
One solution to the higher frequency flutter problem, in the case of digital video signals (or analog signals with suitable conversion), is to store coded signal components in a digital storage buffer at the incoming rate and then retrieve such components for output in the same sequence and at a standard rate. The storage buffer then absorbs the short time variations by, in effect, expanding and compressing in length in accordance with the differentials in signal rates (see e.g. U.S. Pat. No. 3,860,952). This approach, while effective, is expensive and is best suited to broadcast studio systems.
A defluttering approach more suited to consumer video systems employs analog shift registers to remove flutter. With this approach video signals are typically read into registers, one-line-per-register, at the incoming signal rate. Two or more registers receive the signals sequentially and the signal information is read out at the standard rate in the same sequence (see e.g. U.S. Pat. No. 3,931,638). While such an approach is effective for time base correction it does tend to require a great deal of clocking signal and video signal switching. See, also, Fairchild Journal of Semiconductor Progress, Vol. 3, No. 5, Sept., Oct. 1975, pages 16, 17, which discloses a defluttering technique employing input and output registers which sandwich a gate structure adapted for parallel transfer of signals from the input register to the output register.